In an image reading device that reads an image on a document surface, light from a light source extending in a main scan direction is irradiated onto a document surface, reflected light from the document surface is received by a reading sensor such as a CCD, followed by photoelectric conversion, and whereby an image signal is output. Conventionally, in this image reading device, a fluorescence tube such as a xenon tube or a cold-cathode tube is generally used as an illumination device; however, in recent years, in order to respond to market requirements, such as absence of harmful chemical substances or low power consumption, an illumination device that uses a light-emitting diode (LED) as a light source is becoming widespread.
The LED, as a component single body, is small in occupancy area and thus has advantages in freedom of layout and a design freedom of a component to be combined with the LED. However, from another point of view, the LED is small in surface area and is thus low in heat radiating performance as a single body. In addition, while the fluorescence tube radiates most of generated heat as infrared rays, a pseudo-white LED generally used in the image reading device does not emit infrared radiation, resulting in a high heat generation rate in a light-emitting surface. When much current is supplied to the LED so as to obtain high luminance, the heat radiation amount is correspondingly increased. An increase in the temperature of the LED to a predetermined level or more may cause a reduction in light amount, a change in chromaticity, a reduction in the life of the LED and, in the worst case, breakdown of the LED element. When the LED is used to attempt to obtain high luminance output, there is a limit of heat radiation as the LED single body for the reason described above. To solve this problem, there is known an approach in which a metal heat radiation member having both heat conductive property and heat radiation property is tightly fitted to a substrate on which the LED is mounted so as to obtain a heat radiation effect. Further, there is known an image reading device which is provided with heat radiation members on both sides of a substrate on which the LED is mounted so as to further enhance the heat radiation effect (for example, see Patent Document 1).